1. Field of the Invention
The present invention relates to a dual band (mode) portable telephone terminal apparatus and, in particular, to the configuration of an attenuator in a high frequency section inside a radio section of the apparatus. The attenuator according to the invention permits high quality speech through the portable telephone terminal apparatus, and hence is useful as an attenuator in a high frequency section inside a radio section of a dual band (mode) portable telephone terminal apparatus.
2. Related Art of the Invention
In a digital system (such as PDC), the intensity of radio waves delivered from a portable telephone terminal apparatus to abase station needs to be maintained at constant independently of the distance between the portable telephone terminal apparatus and the base station. For this purpose, gain control is performed in a transmitting section in the portable telephone terminal apparatus.
FIG. 9 schematically shows the positional relation between a base station and a portable telephone terminal apparatus. In FIG. 9, the cell region CL of a base station BS has a radius of approximately a few tens km (for example, approximately 30 km). Within the cell region CL of the base station BS, a large number of portable telephone terminal apparatuses TH1 and TH2 are present that have different distance from the base station BS or different communication conditions such as geography. Then, with varying the distance from the base station BS or the communication conditions moment by moment, a large number of these portable telephone terminal apparatuses TH1 and TH2 are simultaneously performing communication with the base station BS.
In such a case, in order that the intensity of the radio waves delivered from a portable telephone terminal apparatus to the base station is maintained to be the same for the nearest location and the farthest location relative to the base station BS within the cell region CL of the base station BS, the transmitting section of the portable telephone terminal apparatus requires a gain control range of approximately 50 dB or wider for the cell region CL of the above-mentioned size. This issue is called the far-near problem.
In case that the gain control is not satisfactorily performed in the transmitting section of the portable telephone terminal apparatus, when the distance of the portable telephone terminal apparatus from the base station decreases, the intensity of the radio waves delivered to the base station increases, and hence causes an increase in the leakage power to adjacent channels. This causes an increase in the code error rate, and hence degrades the speech quality. In FIG. 10, solid lines A1–A6 indicate the intensity of the radio waves of respective channels received at the base station, while a broken line B4 indicates the intermodulation distortion characteristics of the channel A4. FIG. 10 shows the situation that the intensity of the received radio waves of the channels A3 and A5 is buried in the distorted component of the channel A4 indicated by the broken line B4, so that the data is not correctly restored from the channels A3 and A5 adjacent to the channel A4.
In order to maintain a high ratio (C/N) of the carrier signal level to the noise level, the gain control in the transmitting section of the portable telephone terminal apparatus is preferred to be carried out in the high frequency section where the carrier signal is at a high level. This is because in the high frequency section, the carrier signal is at a considerably higher level than the background noise, and hence even when the gain is reduced in the high frequency section, the level difference of the carrier signal from the noise is still maintained to be large. In contrast, when the gain is reduced in the intermediate frequency section where the carrier signal is at a low level, the level difference of the carrier signal from the background noise becomes very small, and this small level difference of the carrier signal from the noise in the intermediate frequency section appears directly in the high frequency section.
For the purpose of achieving such gain control in the range of 50 dB or wider in the transmitting section of the radio section of the portable telephone terminal apparatus, the gain is adjusted by continuous control in the high frequency section and by stepwise control in the intermediate frequency section. Such simultaneous use of the amount of gain control in the high frequency section and the amount of gain control in the intermediate frequency section permits the gain control in the range of 50 dB or wider.
In this situation, the gain control in the portable telephone terminal apparatus is performed as follows. That is, in the portable telephone terminal apparatus, a target value for the transmission power necessary for maintaining the intensity of the received signal at the base station to be constant is set on the basis of the intensity of the received signal at the portable telephone terminal apparatus. Then, a feedback loop is constructed that compares this target value with the actual transmission power so as to cause the transmission power to follow the target value. As such, the gain control is performed such that the transmission power agrees with the target value.
In a dual band (mode) portable telephone terminal apparatus, band (mode) switching is performed when the portable telephone terminal apparatus moves from the cell region CL(A) of a base station BS(A) using a band A (mode A) to the cell region CL(B) of a base station BS(B) using a band B (mode B) as shown in FIG. 11. Here, the bands A and B denote two distinct frequency bands used in the respective cell regions. The modes A and B indicate two distinct systems used there. In this situation, a portable telephone terminal apparatus is first located within the cell region CL(A), and gain control is performed such that the intensity of the radio waves delivered from the portable telephone terminal apparatus to the base station BS(A) is maintained at constant. Then, the portable telephone terminal apparatus moves from the cell region CL(A) into the cell region CL(B). At this instance, band (mode) switching is performed in the portable telephone terminal apparatus, and then gain control is performed such that the intensity of the radio waves delivered to the base station BS(B) is maintained at constant. Symbol TH0 indicates the portable telephone terminal apparatus positioned at the boundary between the cell region CL(A) and the cell region CL(B). Symbols TH1 and TH2 indicate the portable telephone terminal apparatus positioned within the cell region CL(A). Here, the portable telephone terminal apparatus TH1 is at a position near the base station BS(A), while the portable telephone terminal apparatus TH2 is at a position far from the base station BS(A). Symbols TH3 and TH4 indicate the portable telephone terminal apparatus positioned within the cell region CL(B). Here, the portable telephone terminal apparatus TH4 is at a position near the base station BS(B), while the portable telephone terminal apparatus TH3 is at a position far from the base station BS(B).
The configuration and the operation of such a prior art portable telephone terminal apparatus is described below with reference to FIG. 12. As shown in FIG. 12, the portable telephone terminal apparatus is constructed from a microcomputer logic section or the like, and comprises; a baseband section 100 for processing a voice signal; and a radio section 200 for receiving the voice signal processed in the baseband section 100 and then performing communication with a base station.
The radio section 200 comprises: a transmitting section 210 for generating a transmission signal to be transmitted to the base station; and a receiving section 220 for receiving a transmission signal transmitted from the base station.
The transmitting section 210 comprises: an intermediate frequency section 230 for performing the modulation of the voice signal provided from the baseband section 100 and the mixing of the signals for the purpose of frequency conversion; and a high frequency section 240 for band A and a high frequency section 250 for band B, each for amplifying a high frequency signal outputted from the intermediate frequency section 230 and then providing the signal through a switch 310 to an antenna 300.
The intermediate frequency section 230 comprises: a modulator 231; a variable gain intermediate frequency amplifier 232 for amplifying at a variable gain the output signal of the modulator 231; and a mixer 233 for converting the output signal of the variable gain intermediate frequency amplifier 232 into a high frequency signal. The variable gain intermediate frequency amplifier 232 is constructed from a bipolar transistor in many cases. The gain of the variable gain intermediate frequency amplifier 232 can be adjusted discretely in several steps of 5–6 dB intervals. This gain is controlled stepwise in the range of 20 dB or the like by a discrete gain control voltage.
The high frequency section 240 comprises: a variable gain high frequency amplifier 241 for amplifying at a variable gain the high frequency signal of the band A outputted from the intermediate frequency section 230; and a power amplifier 242 for power-amplifying the output of the variable gain high frequency amplifier 241. The gain of the variable gain high frequency amplifier 241 can be adjusted in the range of 40 dB or the like. This gain is controlled continuously in the range of 30 dB or the like by a continuously varying gain control voltage.
The variable gain high frequency amplifier 241 comprises: a preamplifier (medium power amplifier) 244; and an attenuator 243 cascaded to the preamplifier 244 and thereby adjusting the gain for the high frequency signal of the band A to be inputted to the power amplifier (high power amplifier) 242. The attenuator 243 has the function of changing the amount of attenuation in the range of 40 dB or the like.
The high frequency section 250 comprises: a variable gain high frequency amplifier 251 for amplifying at a variable gain the high frequency signal of the band B outputted from the intermediate frequency section 230; and a power amplifier 252 for power-amplifying the output of the variable gain high frequency amplifier 251. The gain of the variable gain high frequency amplifier 251 can be adjusted in the range of 40 dB or the like. This gain is controlled continuously in the range of 30 dB or the like by a continuously varying gain control voltage.
The variable gain high frequency amplifier 251 comprises: a preamplifier (medium power amplifier) 254; and an attenuator 253 cascaded to the preamplifier 254 and thereby adjusting the gain for the high frequency signal of the band B to be inputted to the power amplifier (high power amplifier) 252. The attenuator 253 has the function of changing the amount of attenuation in the range of 40 dB or the like.
The baseband section 100 comprises a controlling section 110. On the basis of the received signal in the receiving section 220, the controlling section 110 determines the band to be used for the high frequency signal to be transmitted, and then applies a drain voltage VDD(A) and a drain voltage VDD(B) to the attenuator 243 and the attenuator 253 respectively, so as to perform band selection for the high frequency signal to be transmitted.
In the communication using the frequency band A, the controlling section 110 detects the signal intensity of the received signal of the receiving section 220, and detects the output level of the power amplifier 242. Then, depending on the signal intensity of the received signal, the controlling section 110 sets a target value for the output level of the power amplifier 242, and then compares the output level of the power amplifier 242 with the target value for the output level of the power amplifier 242, so as to apply again control voltage Vc(RF) corresponding to the comparison result onto the attenuator 243 and apply a gain control voltage Vc(IF) corresponding to the comparison result onto the variable gain intermediate frequency amplifier 232. As such, the amount of the attenuator 243 (the gain of the variable gain high frequency amplifier 241) and the gain of the variable gain intermediate frequency amplifier 232 are controlled and adjusted such that the output level of the power amplifier 242 follows and agrees with the target value for the output level of the power amplifier 242.
Further, in the communication using the frequency band B, the controlling section 110 detects the signal intensity of the received signal of the receiving section 220, and detects the output level of the power amplifier 252. Then, depending on the signal intensity of the received signal, the controlling section 110 sets a target value for the output level of the power amplifier 252, and then compares the output level of the power amplifier 252 with the target value for the output level of the power amplifier 252, so as to apply a gain control voltage Vc(RF) corresponding to the comparison result onto the attenuator 253 and apply a gain control voltage Vc(IF) corresponding to the comparison result onto the variable gain intermediate frequency amplifier 232. As such, the amount of the attenuator 253 (the gain of the variable gain high frequency amplifier 251) and the gain of the variable gain intermediate frequency amplifier 232 are controlled and adjusted such that the output level of the power amplifier 252 follows and agrees with the target value for the output level of the power amplifier 252.
In the portable telephone terminal apparatus described above, simultaneous use of the gain control of the variable gain high frequency amplifier 241 or the variable gain high frequency amplifier 251 and the gain control of the variable gain intermediate frequency amplifier 232 realizes the gain control in the range of 50 dB or wider. In the PDC standard, the input stage of the mixer 233 operates in a 200-MHz band, while the output stage of the mixer 233 operates in a 940-MHz band or a 1441-MHz band. Signal levels in various sections at the maximum output power of the portable telephone terminal are as follows. The output of the power amplifier 242 or the power amplifier 252 has a signal level of +30 dBm (where 0 dBm=1 mW). The output of the variable gain high frequency amplifier 241 or the variable gain high frequency amplifier 251 has a signal level of +8 dBm. The output of the attenuator 243 or the attenuator 253 has a signal level of −16 dBm. The output of the mixer 233 has a signal level of −15 dBm. The output of the variable gain intermediate frequency amplifier 232 has a signal level of −20 dBm.
In this situation, when the variable gain high frequency amplifier 241 performs gain control in the range of 30 dB, and when the variable gain intermediate frequency amplifier 232 performs gain control in the range of 20 dB, the signal level in the output of the variable gain intermediate frequency amplifier 232 varies in the range from −20 dBm to −40 dBm. The signal level in the output of the mixer 233 varies in the range from −15 dBm to −35 dBm. The signal level in the output of the attenuator 243 or the attenuator 253 varies in the range from −16 dBm to −46 dBm. The signal level in the output of the variable gain high frequency amplifier 241 or the variable gain high frequency amplifier 251 varies in the range from +8 dBm to −22 dBm. The signal level in the output of the power amplifier 242 or the power amplifier 252 varies in the range from +30 dBm to −20 dBm.
Examples of such a prior art attenuator include one described in U.S. Pat. No. 4,890,077 and the attenuator 243 (or the attenuator 253) shown in FIG. 13. The attenuator shown in FIG. 13 is different, in detail, from the prior art attenuator described in U.S. Pat. No. 4,890,077. However, in principle, the configuration and the operation of the former are similar to those of the latter.
Specific configuration of the attenuator 243 (or the attenuator 253) and its operation during band selection are described below with reference to FIGS. 13 and 14A–14C.
FIG. 13 is a circuit diagram showing the configuration of the attenuator 243. The attenuator 243 having such configuration performs the gain control. As shown in FIG. 13, the attenuator comprises: a field effect transistor 1 serving as a shunt variable resistor on the input side; a field effect transistor 9 serving as a shunt variable resistor on the output side; capacitors 2, 3, 10, and 11; resistors 5, 7, and 13; and a field effect transistor 6 serving as a series variable resistor.
The attenuator further comprises: a gain control voltage applying terminal 4 for applying the gain control voltage Vc(RF) therethrough; a source voltage applying terminal 8 for applying a supply voltage VDD therethrough; a gate voltage applying terminal 12 for applying a GND terminal voltage (reference potential) therethrough; an input terminal 14 serving as an inputting section for the high frequency signal; and an output terminal 15 serving as an outputting section for the high frequency signal. The input terminal 14 is connected to the output of the mixer 233 of FIG. 12, while the output terminal 15 is connected to the input of the preamplifier 244. The capacitors 2, 3, 10, and 11 block the application of a DC voltage, while the resistors 5, 7, and 13 block the entrance of the high frequency signal.
The attenuator 253 has a circuit configuration (not shown) similar to that of the attenuator 243. It should be noted that the attenuator 243 and the attenuator 253 are formed separately, and hence the field effect transistors incorporated in the attenuators 243 and 253 have certain variation in their characteristics such as the threshold voltage, in some cases.
FIGS. 14A, 14B, and 14C are voltage control characteristics diagrams shown as a function of the location of the attenuator 243 and the attenuator 253. FIG. 14A shows the characteristics of the attenuators 243 and 253 in an overlay manner. FIG. 14B shows the characteristics of the attenuator 243, while FIG. 14C shows the characteristics of the attenuator 253.
The threshold voltage of the series field effect transistor 6 used in the attenuator 243 for the cell region CL(A) is denoted by Vth_T_A, while the threshold voltage of the shunt field effect transistors 1 and 9 is denoted by Vth_S_A. Then, the following relation holds.Vth—T—A=Vth—S—A=−1.8 VWhen the supply voltage applied to the attenuator 243 is denoted by VDD_A, the following relation holds.VDD—A=2.9 V
The gain control voltage that causes the series field effect transistor 6 to go completely off (pinch off) is defined as VcOFF_T_A, while the gain control voltage that causes the shunt field effect transistors 1 and 9 to go completely off (pinch off) is defined as VcOFF_S_A. Then, the following relation holds.Vth—T—A=VcOFF—T—A−VDD—AThus, the following relation is obtained.VcOFF—T—A=Vth—T—A+VDD—A=1.1 VSimilarly, the following relation holds.Vth—S—A=0 V−VcOFF—S—AThus, the following relation is obtained (See FIG. 14B).VcOFF—S—A=−Vth—S—A=1.8 V
Similarly, the threshold voltage of the series field effect transistor used in the attenuator 253 for the cell region CL(B) is denoted by Vth_T_B, while the threshold voltage of the shunt field effect transistors is denoted by Vth_S_B. Then, the following relation holds.Vth—T—B=Vth—S—B=−1.6 VWhen the supply voltage applied to the attenuator 253 is denoted by VDD_B, the following relation holds.VDD—B=2.9 V
The gain control voltage that causes the series field effect transistor to go completely off (pinch off) is defined as VcOFF_T_B, while the gain control voltage that causes the shunt field effect transistors to go completely off (pinch off) is defined as VcOFF_S_B. Then, the following relation holds.Vth—T—B=VcOFF—T—B−VDD—BThus, the following relation is obtained.VcOFF—T—B=Vth—T—B+VDD—B=1.3 VSimilarly, the following relation holds.Vth—S—B=0 V−VcOFF—S—BThus, the following relation is obtained (See FIG. 14C).VcOFF—S—B=−Vth—S—B=1.6 V
FIG. 14A shows an overlay of the characteristics of FIG. 14B and the characteristics of FIG. 14C. In the figure, these attenuation characteristics (threshold values) do not agree with each other. Thus, when the band is switched at the time of move, for example, from the cell region CL(A) into the cell region CL(B), the gain control voltage Vc provided to the attenuator 253 needs to be simultaneously adjusted in order for the output of the attenuator 253 to be set at the same level as the output of the attenuator 243.
The slope of the change in the amount of attenuation is different in FIGS. 14B and 14C. This difference is caused mainly by a difference in the threshold value in the field effect transistors. The difference in the characteristics can be caused also by a variation in the on-resistance in the field effect transistors and by a variation in their parasitic capacitance.
The operation of the attenuator having such configuration is described below. Here, the portable telephone terminal apparatus is driven by a lithium battery or the like at a voltage up to 3.0 V or the like. The threshold voltage of the field effect transistor indicates a bias voltage that causes the variable resistor to begin gain control operation. A ground voltage (reference voltage) is applied to the gate voltage applying terminals 12 of the field effect transistors 1 and 9.
Within the cell region CL(A) where the band A is used, a voltage of 2.9 V is applied to the drain voltage applying terminal 8 of the attenuator 243 in order to select the band A, while a voltage of 0 V is applied to the drain voltage applying terminal 8 of the attenuator 253 in order not to select the band B. When the portable telephone terminal apparatus is located at the position TH1, the distance is the shortest between the portable telephone terminal apparatus and the base station BS(A) within the cell region CL(A). Thus, in order to maximize the amount of attenuation in the attenuator 243, a gain control voltage Vc(RF) of the minimum value (1.1 V) is applied to the gain control voltage applying terminal 4.
Then, with the move of the portable telephone terminal apparatus from the position TH1 to the position TH0, in order to adjust the amount of attenuation of the attenuator 243 from the maximum value to the minimum value, the gain control voltage Vc(RF) applied to the gain control voltage applying terminal 4 is adjusted gradually from the minimum value (1.1 V) to the maximum value (1.8 V).
At the same time when the portable telephone terminal apparatus reaches the position TH0, within the cell region CL(B) where the band B is used, a voltage of 2.9 V is applied to the drain voltage applying terminal 8 of the attenuator 253 in order to select the band B, while a voltage of 0 V is applied to the drain voltage applying terminal 8 of the attenuator 243 in order not to select the band A. At that time, the distance of the portable telephone terminal apparatus (TH0) is the farthest from the base station BS(B) within the cell region CL(B). Thus, in order to minimize the amount of attenuation in the attenuator 253, a gain control voltage Vc(RF) of the maximum value (1.6 V) is applied to the gain control voltage applying terminal 4.
Further, with the move of the portable telephone terminal apparatus from the position TH0 to the position TH4, in order to adjust the amount of attenuation of the attenuator 253 from the minimum value to the maximum value, the gain control voltage Vc(RF) applied to the gain control voltage applying terminal 4 is adjusted gradually from the maximum value (1.6 V) to the minimum value (1.3 V).
On the other hand, in the variable gain intermediate frequency amplifier 232, the output level is adjusted stepwise by changing the gain control voltage Vc(IF) regardless of the selection of the band A or B.
Nevertheless, when the switching from the band A to the band B is performed by the combination of the attenuator 243 and the attenuator 253 fabricated separately, the difference between the threshold voltage (−1.8 V) of the field effect transistors 1, 9, and 6 incorporated in the attenuator 243 and the threshold voltage (−1.6 V) of the field effect transistors 1, 9, and 6 incorporated in the attenuator 253 causes a difference between the gain control characteristics (of the attenuator 243) within the cell region CL(A) and the gain control characteristics (of the attenuator 253) within the cell region CL(B) as shown in FIGS. 14A, 14B, and 14C. In particular, at the time of band selection (at the position TH0), the difference between the gain control voltage (1.8 V) for minimizing the amount of attenuation of the attenuator 243 and the gain control voltage (1.6V) for minimizing the amount of attenuation of the attenuator 253 causes the necessity of a certain time (a few tens μsec or more) for adjusting the gain control voltage into the desired value. In addition, a few tens μsec or the like is necessary from the application of the drain voltage VDD(B) to the achievement of stability in the attenuator 253. Thus, the gain control voltage Vc(RF) applied to the gain control voltage applying terminal 4 of the attenuator 253 delays for this transient response time of the attenuator 253. As a result, a variation in the characteristics of the attenuator 253 and the variable gain intermediate frequency amplifier 232 can cause a difference in the desired gain of the portable telephone terminal apparatus immediately after the band selection.
In such a case, when the portable telephone terminal apparatus is performing communication with moving away from the base station at a constant velocity under ideal conditions, the output Pout of the portable telephone terminal apparatus normally decrease linearly within the cell region CL(B) by virtue of the gain control function, as shown in FIG. 15. Nevertheless, at the time of band selection, the output Pout of the portable telephone terminal apparatus temporarily deviates from the line at the time of band selection because of the delay in the follow operation caused by the time delay in the feedback control and because of the discontinuity in the output level at the band selection. At that time, the intensity of the received signal in the base station deviates from a normal value, so that a level difference occurs relative to the adjacent channel. This has caused the problem of voice disturbance and hence of degradation in the voice quality. This problem has been described for the case that the portable telephone terminal apparatus moves under ideal conditions. Nevertheless, actual conditions during the move are more adverse. For example, the portable telephone terminal apparatus can enter behind a building, so that the intensity of the received signal can decrease suddenly. Thus, the above-mentioned problem should occur frequently that the intensity of the received signal in the base station deviates from a normal value so that the voice quality degrades.
Further, the controlling section 110 of the baseband section 100 needs two kinds of voltage setting corresponding to the two values for the drain voltage VDD(A) and the drain voltage VDD(B) used for the control of the selection and the non-selection of the variable gain high frequency amplifier 241 and the variable gain high frequency amplifier 251. This causes complexity in the control in the controlling section 110.
Furthermore, the high frequency transmitting section 210 needs both the variable gain high frequency amplifier 241 comprising the attenuator 243 and the variable gain high frequency amplifier 251 comprising the attenuator 253. This causes complexity in the circuit configuration, as well as an increase in space requirement. This has caused the problem of an increase in the overall size of the portable telephone terminal apparatus.